1. Field of the Invention
The present invention relates to aqueous preservative solutions with a high amino acid content and cosmetics which contain such a preservative solution.
2. Discussion of the Background
Amino acids or derivatives thereof are useful materials that can be widely used in various cosmetics such as skin cosmetics and hair cosmetics. Due to an increase in the desire for cosmetics made of natural materials in recent years, several kinds of amino acids or derivatives thereof have come to be frequently incorporated therein. However, their quality control or incorporating operation has been extremely complicated. Therefore, an aqueous solution in which versatile and useful amino acids or derivatives thereof are dissolved at a high concentration in advance has been desired. However, in general, such an aqueous solution containing amino acids at a high concentration is easily degraded and deposition is easily caused depending on the amino acids incorporated therein. Therefore, there has been no product which can be used in practical applications.
In general, as a preservation technique, the use of preservatives which can be incorporated in cosmetics such as p-hydroxybenzoic acid esters (parabens) and phenoxyethanol can be exemplified (see, Koshohin Iyakuhin Bofu Sakkinzai no Kagaku). However, there have been problems in that any of the preservatives have an effect only on limited species of microorganisms, respectively, and particularly they show a limited effect on fungi. Further, there have been problems that such preservatives cause irritation to the skin and the like and therefore have a tendency to have low safety, and in the case where products for sensitive skin or made of natural materials are particularly expressly stated, because such preservatives are components which do not exist in nature, an extremely negative impression is given to consumers.
A technique in which preservation is carried out by adjusting the pH of the solution to an acid pH has also been widely known (see, Bokin Bobai Handbook p. 3, Gihodo Shuppan Co., Ltd., 1986). However, when the pH of the solution is 4 or lower, at which an apparent effect is exhibited also on fungi which naturally prefer an acid pH range, the solubility of acidic amino acids is decreased to cause deposition thereof at a low temperature. Therefore, there has been a problem that dissolution stability for achieving distribution of the solution as a product cannot be obtained.
On the other hand, it has been reported that sodium lactate which is also used as a moisturizing agent has a preservative effect (see, Antimicrobial Effects of Lactates: A Review, Journal of Food Protection, vol. 57, no. 5, pp. 445-450 (1994)). However, it was not always effective on fungi that are resistant to osmotic pressure.
Thus, a need remains for an aqueous preservative solution with a high amino acid content which is excellent in preservation, particularly a preservative effect on fungi, and safety, and is excellent in storage stability so as not to cause a problem such as depositing, coloring or smelling even when it is stored for a long time, and by being incorporated in a cosmetic, is capable of imparting a high moisturizing effect without causing a sticky feeling, and imparting an effect of preventing dyed hair from color fading.